Many kinds of coated boards are known in which various kinds of heat-resisting polymers are coated on solid base plates, such as metal plates and ceramic plates.
Typical examples are:
(1) Processes of producing coated films by coating and curing resins containing methylol groups, such as phenol resins, melamine resins, urea resins and furan resins on base plates, by condensation reactions of methylol groups with the elimination of water.
Although coated films of high hardness are obtained from these inexpensive resins, these processes have many disadvantages in that the coated films have only moderate and not very high heat resistivities, polar molecules are likely to remain in the cured films because aldehydes are present in the aqueous solutions or emulsions used as starting materials, the resulting coated film has a disagreeable odor and toxicity problems arise and properties of the films tend to deteriorate upon heating liberating aldehydes.
(2) Processes using epoxy or polyester resins:
These types of resins can be used without employing a solvent and have merits in non-liberation of aldehydes on storage for a long time. However, the curing rate is slow, and the coated films have low resistivity against hydrolysis, tend to liberate a small amount of water with the passage of time, and their thermal stability is not very high but rather is medium.
(3) Processes using aromatic resins such as polyimides or polyimidazoles:
These resins show higher heat resistivities than the resins described in paragraphs (1) and (2), but suffer from such disadvantages as very poor workability, tendency of the solvents used to remain in the cured films, reactivity with certain kinds of metals at elevated temperatures, and easy formation of pinholes on the film surfaces.
(4) Processes using polytetrafluoroethylene or the related polymers:
These kinds of resins have high thermal stability, but the worst disadvantage is the extremely low workability in that formation of a smooth coating surface is very difficult. Some modified kinds of fluoropolymers with improved workability are known, but their heat resistivity and weather resistivity are poorer than those of our invention. Other disadvantages are painting ability and poor adhesive strength to the substrates.
Silicone resins, which have been found wide applications in recent years, show excellent resistivity against heat, weathering, water and chemicals. On the other hand, they suffer from various disadvantages such as low surface hardness, easy heat distortion, poor adhesion and painting abilities, and high moisture permeability. These are due to the intrinsic characteristics of polydimethylsiloxane used as the main component. If polydiphenylsiloxane is used in place of polydimethylsiloxane, some of these properties, such as adhesive and painting abilities, surface hardness and moisture permeability, may be improved. But the rate of curing of polyphenylsiloxanes is slower, and cracks are likely to be formed in the cured films when curing is intentionally accelerated. Because of these troubles, there have been no examples disclosed in which a film-layer is successfully prepared from these types of resins alone.
As described on page 144 of Nakajima and Ariga "Silicone Resins", Plastics Materials Series, Vol 9, (1974) (Nikkan Kogyo Shimbunsha, Japan), for example, it has been generally accepted that a coated film with highest heat resistance can be obtained from a polysiloxane having a phenyl group content of 20-60%; while on the other hand if the phenyl group content exceeds 60%, the hardness of the film may increase, but the rate of polymerization will become slower to cause various problems in processing, such as the formation of only brittle polymers with low molecular weights.